1. Field of the invention
The present invention relates to a circuit for separating luminance and chrominance signals from a composite video signal in a color video processing system, and more particularly to a circuit for separating luminance and chrominance signals using an adaptive 2H comb filters.
Generally, in case of adopting National Television System Committee broadcasting system, using technologies for separating the luminance signal Y and the chrominance signal C from a composite video signal has been proposed in various ways.
Standard color television systems transmit the luminance signal and the chrominance signal separately, and provide luminance signals (Y) that are in phase from horizontal scan line to horizontal scan line and chrominance signals that are 180 degrees out of phase on adjacent horizontal scan lines. At the video level, a composite video signal is established that is Y+C for one horizontal scan line and Y-C for the next horizontal scan line. These sum and difference video signals are utilized to separate the luminance video signal from the chrominance video signal.
Especially, in a motion picture Y/C separation is carried out by using the characteristics between scan lines and in a still picture Y/C separation is carried out by using the frame memory.
The principles of Y/C separation between scan lines which have realized up to now in a motion picture will be described below. The luminance and chrominance signals information of 2 scan lines of FIG. 5 are for example, rely on the contents of the present picture, however between the content of i th line and that of i+1 th line in a motion picture the information of the luminance and chrominance signal may be similar.
If the composite video signal, the luminance and the chrominance signal situated in .DELTA.t from a reference point of the i-th line are designated as Vi, Yi and Ci, respectively and the corresponding signals in the i+1th line are designated as Vi+1, Yi+1 and Ci+1, respectively, the following equation can be made: ##EQU1## Therefore, from the equations (1) and (3) we can obtain the following relationships: EQU Vi(t+.DELTA.t)=Yi(t+.DELTA.t)+Ci(t+.DELTA.t) EQU Vi.sub.1 (t+.DELTA.t)=Yi+.sub.1 (t+.DELTA.t)-Ci(t+.DELTA.t)
Namely, if Yi and Yi+.sub.1 are similar and the chrominance information are similar between adjacent 2 scan lines, Y/C separation can be carried out by adding and subtracting Y+C and Y-C as shown in FIG. 4.
Next, referring to FIG. 6 an adaptive 2H comb filtering method more excellent than the above-mentioned Y/C separation method will be described below. When the number of scan lines are 3 as shown in FIG. 7, Y/C separation of line (5) separates Y/C separation adaptively by detecting the correlation between upper and lower lines. Namely, the differences between line (5) and line (4), and between line (5) and line (6) in FIG. 6 are .DELTA.Y.sub.L +.DELTA.Y.sub.H +2C, respectively. Here, .DELTA.Y.sub.L is a luminance low pass component, .DELTA.Y.sub.H is a luminance high pass component and C is a chrominance signal. If the .DELTA.Y.sub.L .DELTA.Y.sub.H +2C signal passes through low pass filters LPF3 and LPF4, .DELTA.Y.sub.L is obtained respectively, so that the correlation based on the value of .DELTA.Y.sub.L is detected. This means that the smaller the difference component is the larger the correlation is and therefore the chrominance signal C is taken by the difference component between lines with smaller difference component. Here, the luminance low pass correlation is small and the chrominance signal correlation is not detected, because .DELTA.Y.sub.L detected contains only the low pass luminance component.
Further, when there exists no correlation various defects can be caused by carrying out Y/C separation between lines.